In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
Ion implantation is another processing step commonly used in the fabrication of the integrated circuits on the wafer. Ion implantation is a form of doping, in which a substance is embedded into the crystal structure of the semiconductor substrate to modify the electronic properties of the substrate. Ion implantation is a physical process which involves driving high-energy ions into the substrate using a high-voltage ion bombardment. The process usually follows the photolithography step in the fabrication of the circuits on the wafer.
The ion implantation process is carried out in an ion implanter, which generates positively-charged dopant ions in a source material. A mass analyzer separates the ions from the source material to form a beam of the dopant ions, which is accelerated to a high velocity by a voltage field. The kinetic energy attained by the accelerated ions enables the ions to collide with and become embedded in the silicon crystal structure of the substrate. Finally, the doped silicon substrate is subjected to a thermal anneal step to activate the dopant ions.
A phenomenon which commonly results from the ion implantation process is wafer charging, in which positive ions from the ion beam strike the wafer and accumulate in the masking layer. This can cause an excessive charge buildup on the wafer, leading to charge imbalances in the ion beam and beam blow-up, which results in substantial variations in ion distribution across the wafer. The excessive charge buildup can also damage surface oxides, including gate oxides, leading to device reliability problems, as well as cause electrical breakdown of insulating layers within the wafer and poor device yield.
Wafer charging is controlled using a plasma flood system, in which the wafer is subjected to a stable, high-density plasma environment. Low-energy electrons are extracted from an argon or xenon plasma in an arc chamber and introduced into the ion beam, which carries the electrons to the wafer so that positive surface charges on the wafer are neutralized. The energy of the electrons is sufficiently low to prevent negative charging of the wafer.
A typical conventional PFS (plasma flood system) for neutralizing positive charges on ion-implanted wafers is generally indicated by reference numeral 10 in FIG. 1 and includes an arc chamber 12 having a cylindrical chamber wall 14. A single gas inlet opening 18 is provided in the chamber wall 14. A low voltage source 20 generates a typically 3-volt, 200-amp current through a tungsten filament 22 positioned in the chamber interior 13. Pressure inside the chamber interior 13 is maintained at about 5 Torr. Simultaneously, by operation of vacuum pressure applied through a vacuum opening 24 in the bottom of the arc chamber 12, a plasma-forming gas such as argon or xenon is introduced into the chamber interior 13 through the single gas opening 18, at a flow rate of typically about 1.2 sccm. The filament 22, heated by the low-voltage current from the current source 20, causes thermionic emission of electrons from the flowing gas as the gas contacts the filament 22. The electrons from the gas are electrically attracted to the positively-charged chamber walls 14, which function as an anode. A torroidal magnet 16 generates a magnetic field which causes the electrons to travel in a spiral flight path in the chamber interior 13, and this increases the frequency of collisions between the electrons and the gas atoms, resulting in the creation of additional free electrons. The electrons and positive ions are drawn from the chamber interior 13 through the vacuum opening 24, where the electrons and cations enter an ion beam 26. The ion beam 26 carries the electrons into contact with a semiconductor wafer 28 which was previously subjected to an ion implantation process. Accordingly, the electrons contact the wafer 28 and neutralize positive ions remaining on the surface of the wafer 28 after the ion implantation process.
A common characteristic of the conventional arc chamber 12 is that the single gas inlet opening 18 facilitates orderly spiral flow of the argon or xenon gas in the chamber interior 13. Consequently, the plasma-forming gas continually contacts the same point or points on the filament 22 in transit to the vacuum opening 24. This is illustrated in FIG. 2A, in which the flowing gas continually contacts the same point 23 on the filament 22 and, after a relatively short period of operation, causes burnout and breakage of the filament 22 at the point of contact 23. Consequently, the filament 22 must be replaced typically after about 18 days of operation.
As illustrated in FIG. 2B, one way to prevent continuous contact of the gas with the burnout-prone points on the filament 22 is to raise the position of the filament 22 in the chamber interior 13. However, when the filament 22 is disposed in this raised configuration, much of the gas fails to adequately contact the filament 22 for emission of electrons from the gas, as shown by the gas flow path 30.
As illustrated in FIG. 2C, another problem frequently associated with the conventional arc chamber 12 is that the chamber wall 14 at the single gas inlet opening 18 becomes damaged or deteriorated after prolonged use. This causes distortion of the gas flow path 30 to a sharp downward trajectory, such that a large portion of the flowing gas fails to contact the filament 22. Consequently, emission of electrons from the gas is substantially reduced.
It has been found that providing multiple gas entry openings in the chamber wall around the circumference of the arc chamber enhances turbulent flow of the plasma-forming gas in the chamber interior and eliminates continuous contact of the gas with the same point or points on the filament during electron emission. Consequently, the lifetime of the filament is significantly extended.
Accordingly, an object of the present invention is to provide a new and improved arc chamber which prolongs the lifetime of a filament in the emission of electrons from a plasma-forming gas.
Another object of the present invention is to provide a new and improved arc chamber having multiple gas inlet openings for facilitating turbulent flow of plasma-forming gas in an arc chamber of a plasma flood system for ion implanters.
Still another object of the present invention is to provide a multi-inlet arc chamber which prevents premature burnout and breakage of a PFS (plasma flood system) filament in the emission of wafer-neutralizing electrons from a plasma-forming gas flowing into the arc chamber through multiple gas inlet openings.
Yet another object of the present invention is to provide a multi-inlet arc chamber for a PFS system, which arc chamber may have a selected number of multiple gas inlet openings for the introduction of a plasma-forming gas into the chamber.
Another object of the present invention is to provide a multi-inlet arc chamber which may be used to prolong the lifetime of a filament in a plasma flood system for hi-current implanters.
A still further object of the present invention is to provide a method of prolonging the lifetime of a filament used in the emission of electrons from a gas.
Another object of the present invention is to provide a method which may include providing multiple gas inlet openings in an arc chamber in combination with a high gas flow rate to facilitate turbulent flow of a gas through the arc chamber and prevent excessive burnout and breakage of an electrical current filament in the chamber.